1. Field of the Invention
The present invention relates to an optical information reproduction apparatus, and specifically to an optical information reproduction apparatus provided with a semiconductor laser device as a light source which provides oscillations as periodic light pulses upon receiving a DC current. More specifically, the present invention relates to an optical information reproduction apparatus in which a semiconductor laser device functions as a light source with low noise even if laser light reflected from a recording medium is present in the vicinity of the light source.
2. Description of the Related Art
In general, an optical information reproduction apparatus uses a semiconductor laser device as a light source for reading or writing information. In such an optical information reproduction apparatus, laser light emitted by the semiconductor laser device (the light source) is reflected from an optical disk and returns to the semiconductor laser device during reproduction of information, thus generating noise (i.e., so-called "returning light noise"), and thereby causing unstable laser oscillation. There are two well-known methods for reducing such returning light noise: one of them is a high-frequency superimposition method and the other is a self-sustaining pulsation method.
FIG. 1 schematically shows a structure of a driving circuit of a semiconductor laser device 24 used in a high-frequency superimposition method.
More specifically, the driving circuit includes a DC current supply circuit 21 and a high-frequency current superimposition circuit 22. A light output from the semiconductor laser device 24 is monitored by a photodiode 25. A high-frequency current of hundreds of MHz supplied from the high-frequency current superimposition circuit 22 is superimposed on a DC current supplied from the DC current supply circuit 21 through a coupling capacitor 23. The semiconductor laser device 24 is driven with the resultant current to oscillate a high-frequency pulse of hundreds of MHz. Thus, the coherency between the oscillating light inside the semiconductor laser device 24 and the returning light decreases so as to reduce the returning light noise. This prevents the semiconductor laser device 24 to operate with low noise.
On the other hand, according to the self-sustaining pulsation method, a self-sustaining pulsation type semiconductor laser device is used as a light source. More specifically, a semiconductor laser device is driven with a DC current so as to allow a pulse to self-oscillate at a frequency of about 1 GHz. This pulse oscillation decreases the coherency between the oscillating light inside the semiconductor laser device and the returning light so as to reduce the returning light noise. This permits the semiconductor laser device to operate with low noise.
Several structures of a self-sustaining pulsation type semiconductor laser device have been proposed. For example, FIG. 2 is a cross-sectional view showing an exemplary structure thereof.
More specifically, a self-sustaining pulsation type semiconductor laser device 100 shown in FIG. 2 includes an n-AlGaAS cladding layer 112, an AlGaAs active layer 113, a first p-AlGaAs cladding layer 114, and a current blocking layer 115, which are formed on an n-GaAs substrate 111 in this order. The current blocking layer 115 is provided with a stripe-shaped region 120 for serving as a current path. Furthermore, a second p-AlGaAs cladding layer 116 and a p-GaAs cap layer 117 are formed on the current blocking layer 113 in such a manner that the stripe-shaped region 120 is buried under the second p-AlGaAs cladding layer 116. Electrodes 118 and 119 are provided on the top surface of the p-GaAs cap layer 117 and on the bottom surface of the n-GaAs substrate 111, respectively.
In the semiconductor laser device 200 with the above-mentioned structure, the parameters of a device structure are adjusted in such a manner that the amount of a current to be injected into the active layer 113 becomes relatively small with respect to the width of an optical waveguide of a laser cavity. Thus, a self-sustaining pulsation operation can be achieved.
Japanese Laid-Open Patent Publication No. 1-232543 shows that there are high-frequency conditions suitable for reducing noise in the above-mentioned self-sustaining pulsation method. Specifically, it is proposed in the above-mentioned publication that in order to secure low returning light noise characteristics by the self-sustaining pulsation method, the relationship between a frequency of the self-sustaining pulsation and an optical distance from a light-emitting point of a semiconductor laser device to an information recording surface of an optical disk (hereinafter, referred to as an "optical path length") is suitably selected so as not to allow the returning light to be present at the light-emitting point of the semiconductor laser device during rising of a pulse wave.
More specifically, the publication states that in order to sufficiently reduce the returning light noise, it is required to prescribe an optical path length and a frequency of the self-sustaining pulsation so as to satisfy the following Formula (1): EQU n.times.(C/2L)&lt;fs&lt;(n.times.1/2).times.(C/2L) (1)
where L is an optical path length, C is a speed of light, fs is a frequency of the self-sustaining pulsation, and n is 0 or a natural number.
However, according to the conventional high-frequency superimposition method as described above, a high-frequency current generated so as to be superimposed on a DC driving current may leak outside, resulting in noises which prevent various electronic circuits from operating normally. In order to solve such a problem, a shield which prevents leakage of a high-frequency current is required. However, this results in the enlargement of a device. Furthermore, according to this method, since a high-frequency current is superimposed onto a DC current, power consumption increases. These hinder the miniaturization of a device (in particular, the miniaturization of a system using a battery as a power source). In addition, large power consumption also makes it difficult to prolong battery life when a battery is used as a power source.
According to the self-sustaining pulsation method, the reduction of returning light noise is not necessarily achieved as expected.